1. Field of the Invention
The invention relates generally to steering assist apparatuses to allow autonomous vehicle driving along travel lanes. More particularly, the invention relates to a steering assist apparatus for use in a vehicle steering system including right and left steering operation mechanisms that are configured to respectively steer right and left steered wheels and are respectively driven by right and left steering motors when a steering member to be manipulated to steer a vehicle in a desired direction is mechanically coupled to neither of the right and left steering operation mechanisms.
2. Description of the Related Art
With an eye toward implementing a sophisticated driving assist function, typified by autonomous driving, and increasing the layout flexibility of an engine room, the effectiveness of a steer-by-wire system including no intermediate shaft is now being valued highly. To further increase the layout flexibility of an engine room, Japanese Patent Application Publication Nos. 2008-174160 (JP 2008-174160 A) and Japanese Patent Application Publication Nos. 2015-20586 (JP 2015-20586 A) each disclose a right and left independent steering system that is provided with no steering gear device including a mechanism, such as a rack and pinion mechanism, and controls a right steered wheel by a right steering actuator and a left steered wheel by a left steering actuator.
During driving, a driver manipulates a steering member such that a vehicle follows a course as intended by the driver. Making fine adjustments during turning of the vehicle, however, is not easy. One of the reasons for this is that even when the driver finely manipulates the steering member, the vehicle may move to a greater degree than that intended by the driver. Manipulation by the driver may result in a vehicle motion larger than that intended by the driver because tread loads on right and left steered wheels during turning of the vehicle differ from those during straight forward travel of the vehicle, so that a cornering force (CF) applied to the right and left steered wheels during turning of the vehicle does not correspond to what is expected by the driver.
In this regard, a more specific description will be made below. During turning of a vehicle, centrifugal force makes a tread load on an outer wheel greater than a tread load on an inner wheel. More specifically, a tread load on an outer wheel during turning of the vehicle is greater than a tread load on a steered wheel during straight forward travel of the vehicle, and a tread load on an inner wheel during turning of the vehicle is smaller than a tread load on a steered wheel during straight forward travel of the vehicle. Thus, a cornering force applied to an outer wheel is greater than a cornering force applied to a steered wheel when the vehicle is steered at a steering angle during straight forward travel of the vehicle. Accordingly, a motion of the vehicle made in accordance with a change in the steered angle of an outer wheel during turning of the vehicle is more responsive than a motion of the vehicle made in accordance with a change in the steered angle of a steered wheel during straight forward travel of the vehicle. Consequently, a motion of the vehicle made in response to manipulation by the driver during turning of the vehicle is larger than that intended by the driver.
Also in an autonomous driving mode during which a vehicle is autonomously driven by autonomous steering such that the vehicle travels along a travel lane or a set target travel line, a motion of the vehicle made in accordance with a change in the steered angle of an outer wheel during turning of the vehicle is more responsive than a motion of the vehicle made in accordance with a change in the steered angle of a steered wheel during straight forward travel of the vehicle. This may unfortunately reduce the ability of the vehicle to follow the target travel line when the vehicle travels along a curved path.